Bioretention cells include treatment media to remove sediment, debris, chemical pollutants and other contaminants from storm water runoff. Many bioretention cells allow for surface ponding of storm water prior to treatment and include a storage area, usually comprised of washed rock, to detain treated storm water prior to its infiltration into the ground. Hence, a bioretention cell usually serves two purposes: (1) treating storm water runoff for general water quality purposes and (2) allowing the treated runoff to infiltrate into the ground, which reduces runoff rates and total volume of storm water.
While bioretention cells have been installed in a wide variety of landscape applications, typically they must be installed in an area which is not traversed by vehicular or pedestrian traffic. They are not used, for instance, in areas of large, continuous expanses of drivable surfaces, such as parking lots, etc. The task of retrofitting bioretention cells into existing parking lots is also a challenge, because low points in the parking lots are often located with catch basins in the center of paved areas, away from landscaping, which is necessary for effective operation of conventional bioretention cells. Extensive pavement replacement, changes to storm drain arrangements and regrading are often necessary to direct runoff to bioretention cells which are positioned in adjacent landscaped areas.